Few days ago I was hot rodding around in my Model A. On the way back into town I really gave it the gun from a dead stop. It was like a rocketship until about 3rd gear when the whole engine starting chuggling and bucking, almost like it was misfiring or dropping sparks. The rest of the drive home, I couldn't let the RPM under 2000 or it would try to stall. It stalled twice anyways, both when I came to stop and let the RPM get too low. Any idea on what I broke? I'm pretty sure it leaned out and why it did that I'm not sure, right now my fish to fry is getting the engine to run right again. I checked to make sure the carb wasn't screwed up, but I have yet to pull a valve cover or check the timing... My Combo: Stock rebuild 1962 Chevy 283 with stock cam and stock small valve heads Performer RPM intake with 650 vacuum secondary, Holley 140GPH fuel pump with regulator set to 9 psi, 3/8 feed and return lines. Modified Kaiser McCulloch supercharger, 0 psi at idle, 3-4 psi at cruise, 6-7 psi at wide open. Stock single point distributor with a Mallory capacitive discharge box and 60k coil Don't think it matters, but it also has a S-10 T-5 tranny with a 2.76 geared Mopar 8 3/4 and 30" tall tires.
Scotto The supercharger doesn't have a pop off valve right? If it does make sure it isn't stuck open. Really sounds like you got a vacumm leak of some sort. Could you have poped an intake gasket? or poped one of the plugs off a vacumme line? Well maybe the vacumm plugs don't matter on a blow through. Anyway sounds like a vacumme leak. When you get your revs up you are under boost and that shouldn't be a problem. Anyway just my first impression.
I did check the carb plugs and they were all present, that was my first idea, too. Not sure about the intake gasket. I'll have to check on that. I forget to mention, the exhaust was sorta poppin' and a small flame was shooting out of the driver's side header when I was limping it back home. You could see the noise baffle inside the lake header glowing cherry red, too. I couldn't see the passenger side head for comparison though...
Jumped time? You got a good chain and gears on it right? Maybe check the dizzy, sometimes that shear a pin and the gear slips that would make it slow. Thus hot pipes.
Did the temp go up?(lean) Vacuum leak might be it as Pork says. Sounds like it went out of time..timing chain, (gasp)damaged valve. Leak down, if feasible, might indicate an internal problem. Most blowers like a bit of retard as they twist up to avoid detonation. All just guesses..I've run blower motors before but no expert like some of the guys on here. You were talking while I was typing..valve problem or late timing? Header popping is the key.
sounds like you've got a dead hole. fuel isn't getting burned in the combustion chamber and it's lighting off in the exhaust. first thing i'd do is pull the plugs and hopefully the ground strap is still on all of them.
Just went through this on a non supercharged big block chevy. same symptoms. Ended up being a vacuum leak between the carb and manifold. Had the carb off twice before someone sharper than me caught it. So, I'm thinking the pressure found a way out, and now air is finding its way in. In your case, probably intake gasket. Or maybe not, I dunno. Good luck, Ralph
Leaning out/Lean backfiring w/a supercharger and a stock bottom end could result in bad things happening quick. What did your plugs look like, any of them destroyed? I'd check for vacuum leaks like others said, and do a compression test. Check the oil for any chocolate milk in case you blew a head gasket. Basically start with the simplest things and work your way down. Worst case scenario I'd be pulling heads, and pan. Supercharged motors, depending on the amount of boost you are running need to be o-ringed blowing head gaskets can be pretty common if you don't. How does it run above 2000rpm now? Also since it's a "centrifugal" style charge, you need to make sure that carb hat isn't leaking anywhere if you were spitting fire out of the exhaust, and lean backfiring you could have been doing the same out of the carb which in turn could ave blown apart some of the hosing, causing a leak between the super charger and the carb hat, or between the carb/hat and intake.
Just a suggestion - maybe or maybe not directly related to your current problems - but you mentioned you're using a 140 gph Holley pump with a regulator set to 9 lba - and also, that your blower puts s out a maximum of 6 -7 lbs.at WOT. Obviously, since that 's what you've been using, that will work after a fashion , but a much safer set-up would be to use the Holley pump and regulator to feed into a good mechanical pump on the engine - and then run a pressure reference line from your carb hat or airbox to a fitting threaded into the original atmospheric vent hole on the mechanical pump. That way you're guaranteed of always having the fuel pressure referenced to and slighter higher than the boost pressure and prevent any possibility of lean-out at WOT and also eliminate any problems having too much fuel pressure at less than maximum boost. Mart3406 =========================================
Thanks guys! I'll double check for vacuum leaks with some spray and then check the plugs. I have a buddy with a compression/leakdown gauge I can borrow, maybe that will show a dead hole, although I'm hoping it will be something simple.
If you leaned the motor out your plugs will show it for sure. Last time I did that I blew head gaskets.
It's pretty common to pop an intake gasket on a blown small block. It will behave just as you have described. If you had a hole in the piston you'd be blowing oil everywhere, out the breathers, dipstick etc......
Pull the plugs- That will give you a cylinder...... Do the spray- That will confirm gaskets.... Leak down- hope you don't have to go there!
Sure sounds like a vacuum leak. After you check the pop-off to make sure it's not stuck open, take a good look at the base gasket of the supercharger.
About possible lean-out Without a boost referenced mechanical fuel pump set-up like I suggested using in my previous post in this thread, at 7 lbs boost at WOT and 9 lbs.fuel pressure from the electric pump and regulator alone, your actual, *effective* fuel pressure at the carb will only be 2 lbs. which is hardly - or at least, just barely enough - to the carb from going lean at WOT. Even if the boost pressure never exceeds 7 lbs under any condition, 2 lbs. effective fuel pressure at the carb doesn't leave any margin for safety. In a normal, naturally aspirated set-up,a normal mechanical fuel pump will put out 5 or 6 lbs. (over atmospheric pressure) of fuel pressure at the carb. By boost-referencing the pump to the blower output pressure by pressurizing the back side of the pump diaphragm via the original atmospheric vent hole, the pump will now maintain that same 5 or 6 lb. pressure differential - but now, it will be a constant 5 or 6 lbs. over whatever the blower output pressure happens to be, rather than just 5 or 6 lb over atmospheric pressure. Keeping your current electric pump and regulator set up and using it to feed the mechanical pump may not actually be necessary, but since you already have it anyway, using it that way won't hurt and will just give you an extra margin of insurance against ever starving for fuel and going lean. Mart3406 ========================================
Sorry, I have some pictures on digital camera, I was holding off an update until I could post 'em. I checked for vacuum leaks last night, nothing obvious I could see or find with carb spray. Some of the plugs were hotter (whiter) than I'd seen them before, and some of the plugs were white on one side and brownish on the other. After that, I pulled the headers and looked at the valves themselves, they look like they may have gotten hot. Some were white and in the exhaust port of #5 there were white trails leading out of the port. I'll get some pictures up tonight.
Vacume leak or the pop off valve........Had this happen twice and each time it was one or the other....
I understand you logic, but I have to question if it is reltive? I/e, yes a fuel pump will put out 4-6 lbs, but on an NA engine, once the carb is full and the float set, the fuel pump isn't putting nything in. I think the proper way to calculate is to figure out how much fuel is needed (i/e "gallons per hours...etc") and see if the fuel pump is capable of providing the required amount. as an example, just for fodder...I could have a fuel pump putting out a 1000 PSI, through a .005 diameter tube to the carb...fuel would spray or trickle in nd not have enough volume (GPH's) to keep an eninge idling. So, the refernce (IMO) to fuel pump pressure vs. blower pressure is irrelavant. Scotto, sorry I've nothing to to assist you with your engine issues, I've never run a blower. PS...I think you bought a '55 bed from me after the Turlock swap meet several years ago...Mike in Oakdale.
-------------------- It's not irrelevant in a "blow through" application. Carbs work on the principle of pressure differential. In a naturally aspirated situation, fuel in the float bowl is at atmospheric pressure and gets drawn into the venturi by the pressure drop caused by air flowing through the venturi. The faster the air flows through the venturi, the greater the pressure drop and the greater amount of fuel that gets drawn from the float bowl. But when you supercharge the engine by blowing pressurized air through the carb, you change that relationship. The float bowl is still operating at atmospheric pressure, but now, the air going through the venturi is pressurized too. If the air pressure in the carb throat is the same or higher pressure than the pressure acting on the fuel in the float bow, fuel will not be drawn into the venturi. What I suggested re using a mechanical fuel pump and then referencing it's pressure output to boost pressure by pressuring the vent hole is exactly what Paxton did on all their Ford, Studebaker and Kaiser factory installations and what they still do on their current-day aftermarket kits for carbureted engines. and for the exact same reason. Mart3406 =========================================
Thanks!! I love a nice pink donut! Howdy Mike! Okay, I couldn't find any obvious vacuum leaks, so I pulled the plugs and then the headers. The plugs seem whiter than usual, but I guess I really hadn't taken notice before. One interesting thing I noted on some of the plugs was one side of the insulator was whiter than the other. Don't know if that indicates anything. I also attached a couple pictures of valves #3 and #5. #5 had a white trail leading out of the exhaust port I thought was usual. Anyone see anything unusual?
ref imfo post by mart......... the blower pressure seen by the carb on his type install is same all around carb >thus same in the float bowls.........edit needed?
---------------- No, no edit needed. Even with the carb entirely enclosed in a pressure box, you still have to always have the fuel pressure higher than the boost pressure, to maintain the pressure differential that that the carburetor needs in order to draw fuel into the venturi. The purpose of referencing the fuel pump output to boost pressure is so that the fuel pump will always maintain 4 to 6 lbs more pressure than whatever the boost pressure happens to be. Mart3406 =================================================
I see, or think I do ...thanks for clarifying, again, I have no knowledge about blower applications, just struck me odd is all. Gld you did not tke offense, as that was not my intention. .
>>>''the float bowl is still operating at ATMOSPHERIC PRESSURE ect''<<< how so??? carb is in a pressurized hat???
The Jalopy Journal
